Inorg. Chem. 2008, 47,29−41 Experimental and Theoretical Comparison of Actinide and Lanthanide Bonding in M[N(EPR2)2]3 Complexes (M ) U, Pu, La, Ce; E ) S, Se, Te; R ) Ph, iPr, H) Andrew J. Gaunt,*,† Sean D. Reilly,† Alejandro E. Enriquez,† Brian L. Scott,† James A. Ibers,‡ Perumal Sekar,‡ Kieran I. M. Ingram,§ Nikolas Kaltsoyannis,*,§ and Mary P. Neu*,† Chemistry DiVision, Plutonium Manufacturing and Technology DiVision, Materials Physics and Applications DiVision, and Associate Directorate for Chemistry, Life, and Earth Sciences, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, Department of Chemistry, Northwestern UniVersity, 2145 Sheridan Road, EVanston, Illinois 60208-3113, and Department of Chemistry, UniVersity College London, 20 Gordon Street, London WC1H 0AJ, United Kingdom Received August 14, 2007 Treatment of M[N(SiMe3)2]3 (M ) U, Pu (An); La, Ce (Ln)) with NH(EPPh2)2 and NH(EPiPr2)2 (E ) S, Se), afforded the neutral complexes M[N(EPR2)2]3 (R ) Ph, iPr). Tellurium donor complexes were synthesized by treatment of MI3(sol)4 (M ) U, Pu; sol ) py and M ) La, Ce; sol ) thf) with Na(tmeda)[N(TePiPr2)2]. The complexes have been structurally and spectroscopically characterized with concomitant computational modeling through density functional theory (DFT) calculations. The An−E bond lengths are shorter than the Ln−E bond lengths for metal ions of similar ionic radii, consistent with an increase in covalent interactions in the actinide bonding relative to the lanthanide bonding. In addition, the magnitude of the differences in the bonding is slightly greater with increasing softness of the chalcogen donor atom. The DFT calculations for the model systems correlate well with experimentally determined metrical parameters. They indicate that the enhanced covalency in the M−E bond as group 16 is descended arises mostly from increased metal d-orbital participation. Conversely, an increase in f-orbital participation is responsible for the enhancement of covalency in An−E bonds compared to Ln−E bonds. The fundamental and practical importance of such studies of the role of the valence d and f orbitals in the bonding of the f elements is emphasized. 1. Introduction and waste remediation.1-16 In particular, the extent to which covalent contributions are important in f-metal bonding with Two fundamental questions in f-element chemistry are, (1) Roger, M.; Barros, N.; Arliguie, T.; Thue´ry, P.; Maron, L.; Ephri- (1) to what extent do relativistic effects impact the coordina- tikhine, M. J. Am. Chem. Soc. 2006, 128, 8790. tion chemistry of these elements, and (2) what are the (2) Miguirditchian, M.; Guillaneux, D.; Guillaumont, D.; Moisy, P.; Madic, C.; Jensen, M. P.; Nash, K. L. Inorg. Chem. 2005, 44, 1404. bonding differences between 4f and 5f ions of identical ionic (3) Roger, M.; Belkhiri, L.; Thue´ry, P.; Arliguie, T.; Fourmigue´, M.; radii? Addressing these questions is not only a necessity to Boucekkine, A.; Ephritikhine, M. Organometallics 2005, 24, 4940. (4) Denecke, M. A.; Rossberg, A.; Panak, P. J.; Weigl, M.; Schim- acquire a comprehensive understanding of the fundamental melpfennig, B.; Geist, A. Inorg. Chem. 2005, 44, 8418. electronic, structural, and bonding properties of the f elements (5) Mehdoui, T.; Berthet, J.-C.; Thue´ry, P.; Ephritikhine, M. Chem. Commun. (Cambridge) 2005, 2860. but is also highly relevant to industrially important An(III)/ (6) Mehdoui, T.; Berthet, J.-C.; Thue´ry, P.; Salmon, L.; Rivie`re, E.; Ln(III) (An ) actinide, Ln ) lanthanide) separations that Ephritikhine, M. Chem. Eur. J. 2005, 11, 6994. are vital to the development of advanced nuclear fuel cycles (7) Mehdoui, T.; Berthet, J.-C.; Thue´ry, P.; Ephritikhine, M. Eur. J. Inorg. Chem. 2004, 1996. (8) Karmazin, L.; Mazzanti, M.; Bezombes, J.-P.; Gateau, C.; Pe´caut, J. Inorg. Chem. 2004, 43, 5147. * To whom correspondence should be addressed. E-mail: gaunt@ (9) Choppin, G. R. J. Alloys Compd. 2002, 344, 55. lanl.gov (A.J.G.), [email protected] (N.K.), [email protected] (M.P.N.). (10) Jensen, M. P.; Bond, A. H. Radiochim. Acta 2002, 90, 205. † Los Alamos National Laboratory. (11) Berthet, J.-C.; Miquel, Y.; Iveson, P. B.; Nierlich, M.; Thue´ry, P.; ‡ Northwestern University. Madic, C.; Ephritikhine, M. J. Chem. Soc., Dalton Trans. 2002, 3265. § University College London. (12) Jensen, M. P.; Bond, A. H. J. Am. Chem. Soc. 2002, 124, 9870. 10.1021/ic701618a CCC: $40.75 © 2008 American Chemical Society Inorganic Chemistry, Vol. 47, No. 1, 2008 29 Published on Web 11/20/2007 Gaunt et al. soft donor atoms remains unresolved, as does the role of vital for a combined experimental and computational inves- the valence d and f orbitals. One approach to studying this tigation. A relatively small number of studies have observed topic is to crystallographically characterize complexes of 4f shorter actinide-ligand than lanthanide-ligand bond lengths and 5f metal ions, of similar ionic radii, with soft-donor atom with soft-donor atom ligands in structurally similar com- ligands and to use increasingly precise single-crystal X-ray plexes. For example, in the phosphite complexes (MeC5H4)3- diffraction data and computational bonding models to observe ML (M ) U or Ce; L ) P(OCH2)3CEt) the U-P distance is and provide rationale for subtle differences in M-L bond shorter than the Ce-P distance by 0.098 Å.16 In the lengths, specifically to identify and understand differences complexes M(9-aneS3)I3(MeCN)2 (M ) U, La; 9-aneS3 ) in covalency. 1,4,7-trithiacyclononane) the average U-S distance is shorter Obvious functionalities to utilize for studies of covalency than the average La-S distance by 0.0435 Å,14 and in the ) ) are AndL multiple bonds. The bonding of higher oxidation complexes M(tpza)I3(thf) (M U, La; tpza tris[(2- - states (+VI and +V) of some of the early actinides (U, Np, pyrazinyl)methyl]amine) the average U Npyrazine distance is 17 - 13 Pu, and Am) is dominated by the linear actinyl cations that shorter than the average La Npyrazine distance by 0.048 Å. 18 ) contain the multiply bonded dioxo OdAndO moiety, but The comparison of bonding in M(SMes*)3 (M U, La; no high oxidation states for the lanthanides exist for SMes* is a “supermesityl” thiolate ligand) complexes comparison with the actinyl ions. Multiple bonds in lower revealed a U-S distance that is shorter than the La-S oxidation-state actinide ions (mostly U(IV)) occur with distance by an average value of 0.025 Å;1 this represents a sterically bulky and inert stabilizing ligands such as Cp*- rare example in which homoleptic 4f/5f complexes with soft (pentamethylcyclopentadienide).19 However, we are not donors have been compared. Interestingly, Jensen and Bond aware of any isostructural comparisons of MdL multiple conducted an EXAFS solution study to look for differences bonds between 4f and 5f complexes. This is probably a in bond lengths in trivalent Am, Cm, Nd, and Sm complexes reflection of the dearth of MdL multiple bonds reported in with dithiophosphinic acids, which have shown exceptional f-element chemistry, particularly of An(III)dL multiple selectivity for Am(III) over Eu(III) in liquid-liquid extrac- bonds. Therefore, we need to look to other functionalities tion studies.12 However, in that study there was no evidence and systems to assess covalency differences between An and from the EXAFS data of shortened An-S bonds relative to Ln bonding. Ln-S bonds. It may well be that the observed extraction Coordination chemistry studies that directly compare An behavior still was the result of increased covalency in the - - and Ln bonding to soft donor atoms have been inspired to a An S bonds relative to the Ln S bonds but that the bond large extent by solution Ln(III)/An(III) extraction, separation, length differences were too small to be observed experi- and ion exchange experiments, which have demonstrated a mentally. Therefore, to be able to observe bond length preference of soft donor extractants for An(III) ions over differences resulting from covalency, it may be necessary Ln(III) ions.10,12,15,20-23 Understanding the origin of the to use model soft-chalcogen donor complexes judiciously separation and extraction behaviors with soft donors is greatly chosen to maximize covalent character and then to extrapo- aided by the synthesis and characterization of crystallizable late those results to more applied solvent extraction systems. molecular complexes containing soft-donor ligands, which Our approach to elucidating differences in covalency allows structural and geometric data to be obtained that are between An(III) and Ln(III) bonding, as described here, is to undertake an experimental and theoretical comparison of (13) Mazzanti, M.; Wietzke, R.; Pe´caut, J.; Latour, J.-M.; Maldivi, P.; homoleptic, structurally similar An(III) and Ln(III) com- Remy, M. Inorg. Chem. 2002, 41, 2389. plexes in which two important factors are explored: (1) The (14) Karmazin, L.; Mazzanti, M.; Pe´caut, J. Chem. Commum. (Cambridge) electronegativity (or softness) of the ligand donor atom is 2002, 654. (15) Choppin, G. R.; Nash, K. L. Radiochim. Acta 1995, 70-1, 225. varied in order to test the hypothesis that An(III) ions have (16) Brennan, J. G.; Stults, S. D.; Andersen, R. A.; Zalkin, A. Organo- greater covalency in their bonding with soft donor ligands metallics 1988, 7, 1329. (17) See, for example: (a) Burns, C. J.; Neu, M. P.; Boukhalfa, H.; than do Ln(III) ions of identical ionic radii and that the Gutowski, K. E.; Bridges, N. J.; Rogers, R. D. in ComprehensiVe difference in covalency is more pronounced the softer the Coordination Chemistry II, Vol.